Recently, a wireless mobile communication system such as a mobile telephone and a personal handyphone system (PHS) is widely used. In such a wireless mobile communication system, a base station and a relay station for mobile communications RF-amplify a multi-carrier signal including a plurality of carrier waves appropriately modulated at a predetermined frequency interval and then wirelessly transmit the RF-amplified multi-carrier signal. When the linearity of the amplifier used for RF amplification is not sufficiently high, various distortions such as intermodulation distortion are generated.
This distortion prevents the realization of normal or high quality communications. Therefore, high linearity is required for the amplifier used to amplify the multi-carrier signal over the entire frequency band, to which the multi-carrier signal belongs. A feedforward distortion compensation method is known as a method of realizing an ultra-low distortion amplifier for amplifying the multi-carrier signal.
Patent Document 1 discloses a method for controlling a feedforward distortion compensation amplifier capable of suppressing power consumption when power is applied to stabilize an operation.
[Patent Document 1] Japanese Patent Laid-open Publication No. 2004-120545
However, in the conventional technique of Patent Document 1 in particular, when an input signal has a low power level, a sufficiently efficient amplification process is not performed in the range of a distortion level defined by a standard.